Initialize gem
Do the minimal setup

* add a basic description to the gemspec
* add Rakefile with default 'test' task
* add a simple test_helper.rb
Add a prime number sieve

A simple class optimized for speed that allows to get the first n prime
numbers as well as to only get the nth one.
Bring the multiplication table to life

* implement MultiplicationTable
** allow an arbitrary number range to be set across and down
** fill the table with the products of the respective rows and columns
** use Matrix as data structure

* implement MultiplicationTable#to_s
** print headers across and down
** right-align the numbers while making sure there's always enough space
Add a simple CLI

* provide option 'size' to set the multiplication table's size (default
  to 10)
* add 'number' option to get the nth prime number
* make sure to raise respective errors if arguments aren't numerical